As crosslinkers for aliphatically unsaturated organopolysiloxanes, use is made virtually exclusively of organopolysiloxanes containing hydrogenmethylsiloxane units, in the simplest case hydrogenmethylpolysiloxane end-blocked by triorganosiloxy groups. To increase the reactivity, it has proven useful to incorporate dimethylsiloxane units between the hydrogenmethylsiloxane units by equilibration. However, these measures are only very limited improvements.
Apart from these crosslinkers based on hydrogenmethylsiloxane units, crosslinkers without methyl groups are also known. DE-B 1 955 511 and the corresponding U.S. Pat. No. 3,615,272 describe resins comprising hydrogensiloxane units, i.e. trifunctional units. However, for use as crosslinkers in addition-curing organopolysiloxane compositions, such polymers are only of very limited use, since they have a very low solubility in the crosslinkable diorganopolysiloxanes.
EP-A 568 318 discloses organopolysiloxanes comprising trifunctional siloxane units and monofunctional siloxane units containing Si-bonded hydrogen. According to EP-A 568 318, T units in the form of organosiloxane units are end-blocked with hydrogendimethylsiloxy groups, with a different number of dimethylsiloxane or hydrogenmethylsiloxane units being able to be incorporated between these groups.
According to DE-A 37 16 372, organopolysiloxanes containing Si-bonded hydrogen atoms and from 3 to 5 silicon atoms per molecule are prepared from [H(CH.sub.3).sub.2 Si].sub.2 O and trialkoxysilanes in a hydrolytic process.
DE-A 195 22 144 (Wacker-Chemie GmbH, published on Jan. 2, 1997) discloses polyaddition products comprising hydrogendimethylsiloxane units which are prepared by reacting hydrocarbons having at least three terminal double bonds with short-chain .alpha.,.omega.-dihydrogensiloxanes.
DE-A 196 02 663 (Wacker-Chemie GmbH, published on Jul. 31, 1997) describes carbosiloxane crosslinkers having terminal units of the formula H.sub.a R.sub.3-a SiO.sub.1/2 (where a is preferably 1 and R is a monovalent hydrocarbon radical) and carbostructural units which may comprise units of the formula H.sub.c R.sub.d SiO.sub.4-c-d/2 (where c is preferably 0 and the sum c+d is preferably 2), i.e. they can also have bifunctional siloxane units. An essential feature of these carbosiloxane crosslinkers is that the total number of Si-bonded hydrogen atoms present in the terminal units is greater than the total number of all Si atoms connected to one another via the carbo-structural unit.